1. Field of the Invention
The present invention relates to control devices for internal combustion engines, and more particularly relates to a control device for an internal combustion engine that improves fuel consumption of the internal combustion engine.
2. Description of the Background Art
In an internal combustion engine, exhaust gas recirculation amount, for example, has such a correlation with fuel consumption and combustion variation as follows. As the exhaust gas recirculation amount increases, the fuel consumption of the engine gradually becomes better, and at a certain exhaust gas recirculation amount, a value is reached at which the fuel consumption is minimized (hereafter referred to as a “fuel-consumption optimum value”), and if the exhaust gas recirculation amount is increased further, the fuel consumption worsens. Meanwhile, as the exhaust gas recirculation amount is increased, the combustion condition of the engine gradually becomes smaller, but when it exceeds a certain exhaust gas recirculation amount, a combustion variation abruptly increases, and it exceeds a limit value (hereafter referred to as a “combustion variation limit value”) under which the combustion variation can be permitted. Moreover, the fuel-consumption optimum value and the combustion variation limit value are attained at approximately a same exhaust gas recirculation amount.
The correlation of ignition timing with the fuel consumption and the combustion variation also shows a similar correlation to the above-described correlation of the exhaust gas recirculation amount with the fuel consumption and the combustion variation.
The correlation of air-fuel ratio with the fuel consumption and the combustion variation also shows a similar correlation to the above-described correlation of the exhaust gas recirculation amount with the fuel consumption and the combustion variation.
Therefore, it is understood that the fuel consumption will be minimized if manipulation values are set for the exhaust gas recirculation amount, the ignition timing, and the air-fuel ratio such that the combustion variation results in the combustion variation limit value.
However, it is difficult to set such manipulation values for the exhaust gas recirculation amount, the ignition timing, and the air-fuel ratio engine as to obtain the combustion variation limit value taking into account individual variations among internal combustion engines and their degradations over time. For example, if the exhaust gas recirculation amount is set at a manipulation value such as to result in the combustion variation limit value with a certain internal combustion engine, the combustion variation may increase and exceed the combustion variation limit value with another internal combustion engine in the same operating condition, because exactly the same combustion condition will not occur because of their individual variations.
For that reason, those manipulation amounts are allowed to have a certain degree of additional margins (hereafter referred to as “margins”). Nevertheless, by allowing the exhaust gas recirculation amount to have a margin, the fuel consumption becomes greater than the fuel-consumption optimum value.
Likewise, the ignition timing is also allowed to have a margin for the same reason as the exhaust gas recirculation amount, but by allowing the ignition timing to have a margin, the fuel consumption becomes greater than the fuel-consumption optimum value.
Likewise, the air-fuel ratio is also allowed to have a margin for the same reason as the exhaust gas recirculation amount, but by allowing the air-fuel ratio to have a margin, the fuel consumption becomes greater than the fuel-consumption optimum value.
Thus, although the fuel consumption is increased, variations among internal combustion engines and their deteriorations over time are absorbed and an operation with a small combustion variation in the internal combustion engines is ensured by allowing the exhaust gas recirculation amount, the ignition timing, and the air-fuel ratio to have margins.
Nevertheless, improvement in the fuel consumption of internal combustion engines has been demanded increasingly in recent years, and various patent applications have been made.
For example, Japanese Patent Application Laid-open No. 5-215004 (1999) discloses a technique in which, by detecting a combustion variation in an internal combustion engine and increasing the exhaust gas recirculation amount until the detection result is determined to be an abnormal combustion, the combustion condition is controlled to result in the maximum exhaust gas recirculation amount within the combustion variation limit to reduce the amount of the fuel consumption increased due to the margin of the exhaust gas recirculation amount.
In addition, Japanese Patent Publication No. 8-30461 (1999) discloses a technique in which, by detecting knocking in an internal combustion engine and advancing the ignition timing until knocking occurs, the combustion is controlled with the ignition timing of the maximum advance angle at which knocking does not occur to reduce the amount of the fuel consumption increased due to the margin of the ignition timing.
In addition, in Japanese Patent Application Laid-open No. 9-96238 (1999), a technique is disclosed that optimizes engine cylinder internal pressure to improve the fuel consumption by detecting an engine cylinder internal pressure and manipulating the air-fuel ratio and the ignition timing so that the engine cylinder internal pressure is maximized. Moreover, when the combustion temperature of the engine is higher than a predetermined value, the exhaust gas recirculation amount is manipulated to lower the combustion temperature, thus suppressing the emission of NOx, which is one component of exhaust gas.
The technique of Japanese Patent Application Laid-open No. 5-215004 has a problem that although the amount of the fuel consumption increased is reduced by merely increasing the exhaust gas recirculation amount to attain a combustion variation limit value in an exhaust-gas-recirculation-amount increasing direction, the fuel consumption does not reach the real fuel-consumption optimum value and becomes greater than the real fuel-consumption optimum value.
Likewise, the technique of Japanese Patent Publication No. 8-30461 has a problem that although the amount of the fuel consumption increased is reduced by merely advancing the ignition timing to attain a combustion variation limit value in the ignition timing advancing direction, the fuel consumption does not reach a real fuel-consumption optimum value and becomes greater than the real fuel-consumption optimum value.
Further, in Japanese Patent Application Laid-open No. 9-96238, the fuel consumption is improved by advancing ignition timing and moreover NOx is suppressed by increasing the exhaust gas recirculation amount when the combustion temperature is higher than a predetermined value. However, there is another problem that when the combustion temperature is less than the predetermined value, the exhaust gas recirculation amount is not manipulated in the increasing direction, so the fuel consumption does not reach the real fuel-consumption optimum value and becomes greater than the real fuel-consumption optimum value.
Another problem has been that the fuel-consumption optimum value cannot be reached quickly when the combustion temperature is higher than a predetermined value.